Laser ultrasonic imaging

Abstract: The invention relates to method and apparatus for optically detecting transient motion from a scattering surface. A broad laser beam (10) is directed onto an area of the scattering surface (14) of an object to be imaged to thereby scatter the laser beam and produce a plurality of modulated signals. A demodulator (16) simultaneously demodulates the modulated signals to provide a plurality of demodulated signals to an array or matrix of detectors (18). The detector array simultaneously converts the demodulated light signals into electrical signals representative of the transient motion. The optical detection technique according to the invention is particularly useful for detecting small deformations produced by ultrasound, in a timely manner by parallel processing information provided by a matrix of detectors.

Description: The invention generally relates to a method and apparatus for optically detecting transient motion from a scattering surface. More particularly, the invention relates toward detecting optical phase modulations such as those produced by ultrasound and producing an image of this motion over the surface as well as producing a ultrasonic image of the object excited by ultrasound. The detection of phase modulation or frequency modulation of an optical wave is important from various fields of application where optical beans are used to detect the motion of objects. This is the case of laser sensing of vibrations and laser detection of ultrasound and of transient body deformations such as those produced by a shock or impact. Of particular interest for practical applications is the case where ultrasound or a shock wave is generated by a laser. In this case, a completely remote ultrasonic inspection system can be realized, permitting for example ultrasonic probing at elevated temperatures. A technique base on laser generation and optical detection can thus be advantageously used to inspect materials at high temperatures (such as all metals and ceramics) for process and quality control, to detect flaws as soon as they are created during processing, to measure production parameters such as thickness, temperature, etc. and to determine micro structural properties on-line (grain size, porosity, etc.) This technique is also particularly advantageous to inspect parts of complex shapes such as those made of polymer matrix composite materials and used in advanced aeronautic and aerospace structures. In all cases of practical interest, ultrasonic excitation of the object produces at its surface, very small displacements that translate into correspondingly very small phase or frequency perturbations. Therefore, a sensitive detection technique has to be used, which means in practice a technique based on optical interferometry. Since in practice, the probed surfaces arc rough, the ultrasonic information is encoded into a optical beam with speckle and a suitable interferometric technique should integrate effectively over the whole speckle field or provide demodulation independently of speckle nature of the collected light beam. In various U.S. patents, Applicant has described interferometric schemes for sensitive detection in these conditions. All of these schemes are characterized by their large étendue parameter. This étendue parameter (or throughput), is defined as the product of its effective entrance aperture area by the solid angle limited by the laws of maximum inclination passing through the entrance aperture centre and thus defining the field of view. The maximum inclination rays can be defined as those which produce a shift of the interference pattern by a quarter of a fringe. The importance of the étendue parameter stems from its invariance within the frame of geometric optics. A large étendue permits to choose light collecting optics of large size, being only limited by cost and practical feasibility, and to detect surface motion over a large area. In an arrangement described by the applicant in U.S. patent number 4,659,224 issued April 21, 1987, entitled Optical Interferometric Reception of Ultrasonic Energy, a confocal Fabry-Perot is used in transmission to provide a signal representative of the surface motion independently of the speckle effect. In U.S. patent number 4,966,459 issued October 30, 1990, entitled Broadband Optical Detection of Transient Surface Motion From a Scattering Surface, the applicant describes the use of the same type of interferometer, that may be used within a Mach-Zehnder interferometric arrangement or in a reflection scheme to provide the same capability with a very broad detection bandwidth. Still a broader detection bandwidth, especially including the low ultrasonic frequency range, several KHz to about 1 MHz, is described by the applicant and R.K. Ing in U.S. patent number 5,131,748 issued July 21, 1992, entitled Broadband Optical Detection of Transient Motion from a Scattering Surface that is based on the use of two-wave mixing in a photorefractive crystal. Although the prior art described performs its intended function adequately, it only provides information relating to a single location on the surface of the object. In order to obtain an ultrasonic image of the object representative of a property of interest, such as thickness, porosity, microstructure, presence and location of flaws, the detection laser beam is usually scanned with a rotating mirror over the surface of the object. Alternatively the object can be moved in front of the laser beam. Although such an approach works properly, it is slow and can take precious time if a large surface area is to be scanned. The present application describes a method and apparatus that can generate an ultrasonic image of the object without using optical scanning and can simultaneously detect signals originating from multiple spots on the surface of an object. It is an object of the invention to seek to overcome this limitation by providing a method and apparatus for simultaneous detection of a plurality of signals representative of the transient motion of an excited object. US-A-4905202 describes a detection panel for acoustic holography with real time image reconstruction. Ultrasonic motion from the surface of a liquid crystal is determined by reflecting a laser beam from a reflective surface of the liquid crystal. The liquid crystal provides a refractive index which varies as a function of acoustic pressure. Applied Optics vol. 20, no. 8, 15/4/1081 NY US, pages 1467-1470, H.J. Tiziani et al. "Vibration analysis by speckle techniques in real time" describes a method of quasi-real time vibration analysis by recording the time-averaged speckle pattern in a bismuth silicon oxide crystal. Optics Communications, vol. 96, no. 4-6, 15/2/1993 Amsterdam NL, pages 195-201, Q.-C.-Zhang et al. "Multiple dynamic speckle correlation method with a LCTV panel" describes a multiple dynamic speckle correlation method with a LCTV panel. According to one aspect of the invention an apparatus for detecting transient motion from a scattering surface is provided. The apparatus comprises means for generating a broad beam of coherent light onto the scattering surface to thereby scatter the laser beam and produce a plurality of signals in the form of modulated signals; means for simultaneously demodulating a plurality of the modulated signals to produce demodulated signals which are representative of the transient motion; and, an array of detectors for simultaneously detecting the demodulated signals and for converting them into electrical signals representative of the transient motion. According to another aspect of the invention a method is provided for detecting transient motion from a scattering surface. The method comprises the steps of directing a broad laser beam onto the scattering surface to thereby scatter the laser beam and produce a plurality of signals in the form of modulated signals; simultaneously demodulating a plurality of the modulated signals to produce demodulated signals which are representative of the transient motion; and, simultaneously detecting the demodulated signals with an array of detectors and converting them into electrical signals representative of the transient motion over parallel channels. By the expression "transient motion" it is meant a motion having a non-zero acceleration. Such an expression thus includes all oscillating motions, but excludes motions of constant velocities. This invention is particularly useful for rapid imaging of small surface deformations or displacements of a material subjected to ultrasonic energy. Exemplary embodiments of the invention will now be described in conjunction with the drawings in which: FIG. 1 is a schematic block diagram illustrating a first embodiment of an imaging apparatus including an array of detectors in accordance with the invention; FIG. 2 is a schematic block diagram illustrating a second embodiment of an imaging apparatus including optical fibers for guiding signals to an array of detectors in accordance with the invention; FIGs 3a and 3b are end views of closely packed optical fibers; FIG. 4 is a schematic block diagram of an embodiment of imaging apparatus having an array of photorefractive crystals, in accordance with the invention; FIG. 5 is a schematic block diagram of an embodiment of imaging apparatus having an array of photorefractive crystals, similar to the embodiment shown in FIG. 4, however, having electrical leads attached for carrying detection signals, in accordance with the invention; FIG. 6 is a schematic block diagram of an embodiment of imaging apparatus having optical fibers for guiding signals to an array of photorefractive crystals. FIG.s 7a and 7b are a prior art schematic diagrams of imaging apparatus having a Fabry-Perot arrangement to provide a signal representative of transient motion on the surface of a workpiece; and, FIG. 7c is a prior art schematic diagram of imaging apparatus having a nonlinear or photorefractive crystal as a demodulator. Referring first to FIG. 1, a broad laser beam 10 generated from a laser source 12 is directed onto a surface 14 of a material or workpiece subjected to ultrasonic energy. The ultrasonic displacement of the surface 14 probe by the broad laser beam 10 can be produced by an ultrasonic piezoelectric transducer or other means such is electrical discharge, projectile impact or high intensity laser pulse; it can also occur naturally in strained material. As the broad laser beam 10 having an area α impinges on the surface 14, it is scattered by the surface, the scattered laser beam thus produced being phase or frequency modulated by the ultrasonic surface motion of the object. A demodulator 16 in the form of a detecting interferometer with a large étendue demodulates the frequency or phase shift produced by the ultrasonic motion. The demodulator block 16 may be a two-way type or a multi-wave type. A paper entitled Optical Detection of Ultrasound published in the IEEE Transactions on Sonics, Ultrasonics and Frequency Control, UFFC-33, pp. 485-499 (1986), describes the two-wave type of interferometer. The confocal Fabry-Pérot interferometer is described by the applicant in US patent No. 4,659,224 entitled "Optical Interferometric Reception of Ultrasonic Energy" and in U.S. patent No. 4,966,459 entitled "Broadband Optical Detection of Transient Surface Motion from a Scattering Surface". Similarly an interferometric arrangement based on two-wave mixing in a photorefractive or nonlinear crystal can be used. A suitable configuration has been described in U.S. patent number 5,131,748 issued July 21, 1992, entitled Broadband Optical Detection of Transient Motion from a Scattering Surface by Two-Wave Mixing in a Photorefractive Crystal. One embodiment of this system is shown in FIG. 7c. Alternatively, FIG.s 7a, and 7b show two other prior art systems having confocal Fabry-Pérot interferometers. Referring again to FIG. 1, and for the purpose of illustration, only two points of an infinite number of points on the surface within the area a of the broad beam 10 are shown. Detection means in the form of an array of detectors are positioned behind the demodulator 16 to receive demodulated signals provided by the demodulator. Each detector 18 in the array, detects information relating to a particular location or sub-area within the surface area a . This is somewhat analogous to an image recorded by a camera stored into a discrete number of pixels. In this mention, an area a of the scattering surface 14 is illuminated by the broad beam laser 12. The resultant scattered beam containing information relating to the surface 14 of area a is demodulated and an array of n detectors simultaneously detects information relating to n separate subareas within the illuminated area a . Advantageously, the array of detectors 18 provides means for simultaneously detecting and converting into electrical signals the demodulated information bearing signals. Since the illuminating broad laser beam 10 may not have a perfectly uniform intensity distribution and since the light reflection or scattering properties of the surface 14 may vary across the observed area a , electronic circuitry is used to normalize the signal of each detector to the amount of light received by the detector. This can be conveniently performed by using capacitive coupling to separate the DC output of the detector representative of the collected light from its AC output. Both outputs can be digitally sampled. After division of the AC signal by the DC signal, a normalized signal, representative of the surface transient or ultrasonic motion is obtained. Alternatively, the laser beam can be phase modulated at a given modulation frequency and transient surface motion signal can be separated from the additional phase modulation signal with suitable RF filters after detection. This is described in U.S. patent 4,659,224 issued April 21, 1987, entitled Optical Interferometric Reception of Ultrasonic Energy. The additional modulation signal is then rectified to provide a DC signal to be used for normalization as described above. In FIG. 1, the optical system includes first and second lenses 6 and 8 respectively for focusing or steering the scattered beam toward the detector 18. The first lens 6 is positioned Intermediate the scattering surface 14 and the demodulator 16; the second lens 8 is positioned intermediate the detector array 18 and the demodulator 16. The scattered beam comprising a plurality of composite signals bearing information relating to the entire area a of the surface 14 is demodulated and presented to the detector array or matrix. A unique signal representative of the transient motion of a particular sub-area on the scattering surface is presented to each detector and is processed independently of the other detectors; thus, the arrangement shown in FIG. 1 allows parallel processing or simultaneous detection of a plurality of signals. Of course, the detection resolution increases as the number n of detectors and number n of sub-areas in the area a a increases. Since there is a one-to-one mapping between sub-area or spot within the area a and a detector, decreasing the size of the spot that is mapped to each detector in effect yields a more precise result with more information per unit of area. This would be analogous to decreasing the pixel size in a scanned image to provide a higher definition image. Now referring to FIG. 2 a broad laser beam 10 generated from a laser source 12 is directed onto the surface of a material or workpiece 14 subjected to ultrasonic energy. The arrangement shown in FIG. 2 is similar to that of FIG. 1 however, two lenses 24 and 25 are positioned intermediate the demodulator 16 and the workpiece 14. The lenses serve as a focusing means to assist in guiding the signals to the demodulator 16. Optical wave guide means in the form of a pair of optical fibers are located intermediate lenses 24 and 25 and provide a suitable transmission medium for signal, to be carried to the demodulator. For the sake of clarity, only two optical fibers have been shown; however, in a preferred arrangement, a plurality of fibers in excess of two is provided for carrying the same number of channels to a corresponding number of detectors 18. A lens 26 is positioned between the array or matrix of detectors 18 and the interferometric demodulator 16. A second pair of optical fibers located intermediate detectors D 1 and D 2 and the lens 26 receive signals representative of the transient motion on the surface of the workpiece 14. Bundles of optical fibers or optical wave guides may be particularly convenient in an industrial environment where the laser source, the demodulating interferometer and/or the detectors or inspection area may be far away from one another. Furthermore, the optical fibers may be closely packed as shown in FIGs. 3a and 3b for providing direct coupling (not shown) between the workpiece and the demodulator. Close-packing is advantageous since any interferometric detection system has limited étendue. Optical fibre coupling in front of the interferometer provides for widely separated probed spots without increasing the étendue of the beam at the interferometer entrance. In addition, optical fiber coupling after the demodulator allows the geometrical arrangement of the detectors within the array or matrix to be completely independent of the geometrical arrangement of the detection channels inside the demodulator or the spots on the surface. FIG. 4 shows another embodiment of the invention where the detecting interferometer has been segmented in as many elements as detection channels. This can be implemented by using two-wave mixing on a photorefractive wafer upon which separate zones have been etched or by using an ensemble of small photorefractive crystals 42 or nonlinear crystals. As shown in FIG. 4, all these crystals are illuminated by the broad area pump beam directly derived from the laser source 12. These cystals are then imaged onto the detector array or matrix 18. I. A. Sokolov, S.I. Stepanov and G.S. Trofimov, in the Journal Opt. Soc. Am B/Vol. 9, No. 1/January 1992, p.p. 173-176 disclose the detection of a small alternating current through a short circuited sample of a photorefractive crystal that is illuminated by a vibrating pattern of interference between two waves. The current produced corresponds to the optical phase modulation in the information bearing signal contained within the interference pattern illuminating the photorefractive crystal. Referring now to FIG. 5, an array of photorefractive crystals 52 is shown in accordance with the invention in an alternative embodiment. The use of several photorefractive crystals each having a lead attached for carrying current, allows simultaneous demodulation and simultaneous detection of a plurality of signals. In this arrangement, each crystal demodulates and detects a signal corresponding to a particular sub-area within the area a on the scattering surface. Of course, an embodiment using an optical fiber link as shown in FIG. 6 provides additional flexibility providing probed spots distributed on the probed surface differently from the detecting elements.

Claims: An apparatus for detecting transient motion from a scattering surface comprising: means for generating a broad beam of coherent light onto the scattering surface to thereby scatter the laser beam and produce a plurality of signals in the form of modulated signals; means for simultaneously demodulating a plurality of the modulated signals to produce demodulated signals which are representative of the transient motion; and, an array of detectors for simultaneously detecting the demodulated signals and for converting them into electrical signals representative of the transient motion. An apparatus as defined in claim 1, wherein each detector is configured to detect demodulated signals independent of the other detectors. An apparatus as defined in any of the preceding claims, wherein the means for simultaneously demodulating is in the form of a plurality of segmented demodulators. An apparatus as defined in claim 3, wherein the segmented demodulators are in the form of non-linear crystals. An apparatus as defined in claim 4, wherein the non-linear crystals are photorefractive crystals. An apparatus according to any of the preceding claims, further including optical wave guide means intermediate the scattering surface and the demodulating means. An apparatus as defined in claim 6, wherein the optical wave guide means comprises a plurality of optical fibres, each of the fibres having an end proximate to the scattering surface and an other end proximate to the demodulating means. An apparatus according to any of the preceding claims, wherein the means for demodulating and the detector means are integrated whereby demodulation, detection and conversion into electrical signals are performed simultaneously. An apparatus as defined in claim 8, wherein the integrated means for simultaneously demodulating and for simultaneously detecting and converting into electrical signals comprises a plurality of non-linear crystals. An apparatus as defined in claim 9, wherein the means for simultaneously demodulating and for simultaneously detecting comprises a plurality of photorefractive crystals. A method for detecting transient motion from a scattering surface comprising the steps of: directing a board laser beam onto the scattering surface to thereby scatter the laser beam and produce a plurality of signals in the form of modulated signals; simultaneously demodulating a plurality of the modulated signals to produce demodulated signals which are representative of the transient motion; and, simultaneously detecting the demodulated signals with an array of detectors and converting them into electrical signals representative of the transient motion over parallel channels.

Inventors: MONCHALIN JEAN-PIERRE, MONCHALIN, JEAN-PIERRE
Applicants: CANADA NAT RES COUNCIL, NATIONAL RESEARCH COUNCIL OF CANADA
Application year: 1994
Main classification: G01H9
IPC codes: G01D5, G01H9